Methods and compositions for inhibiting vinyl aromatic monomer polymerization

ABSTRACT

Methods and compositions are provided for inhibiting the polymerization of a vinyl aromatic monomer, such as styrene monomer, during elevated temperature processing thereof or during storage or shipment of polymer containing product. The compositions comprise a combination of a quinone methide derivative A) and a phenol compound B). The methods comprise adding from about 1-10,000 ppm of the combination to the monomer containing medium, per one million parts of the monomer.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional of allowed U.S. patentapplication Ser. No. 12/793,226 filed Jun. 3, 2010.

FIELD OF THE INVENTION

The invention pertains to methods and compositions for inhibiting theundesired polymerization of vinyl aromatic monomers, such as styrenemonomer, during processes such as monomer preparation, and purification,and during storage and shipment of products containing such monomers.

BACKGROUND OF THE INVENTION

Polystyrene is a thermoplastic with many desirable characteristics. Itis clear, transparent, readily colored and easily fabricated. The familyof styrene polymers includes polystyrene itself, copolymers of styrenewith other vinyl monomers, polymers of derivatives of styrene andmixtures of polystyrene and styrene-containing copolymers withelastomers.

ABS (acrylonitrile, butadiene-styrene) resins have enjoyed tremendouscommercial popularity for many years as durable, temperature and solventresistant elastomers. On the other hand, styrene plastics are commonlyused for packaging, including foams and films, coatings, in appliancefabrication, for housewares and toys, lighting fixtures and inconstruction materials.

It is well known that styrene monomer readily polymerizes when heated orexposed to light. Heat polymerization is rapid. In fact, polymerizationincreases with increasing temperature. This polymerization isundesirable during many stages of the manufacturing, processing,handling, storage and use of styrene monomers.

Common industrial methods for producing styrene include a variety ofpurification processes, including distillation, to remove impurities.Unfortunately, purification operations carried out at elevatedtemperatures result in an increased rate of undesired polymerization.Polymerization, such as thermal polymerization, during the monomerpurification process, results not only in loss of desired monomer end-product, but also in loss of production efficiency caused by polymerformation or agglomeration on process equipment. In heat requiringoperations, such agglomeration adversely affects heat transferefficiency.

SUMMARY OF THE INVENTION

In accordance with aspect of the invention, a method is provided forinhibiting the polymerization of a vinyl aromatic monomer such asstyrene monomer, i.e., ethylbenzene. The method comprises adding aneffective polymerization inhibiting amount of a combined treatment tothe monomer medium. The combined treatment comprises (A) a quinonemethide derivative and (B) a phenol compound. From about 1-10,000 ppm of(A) and (B) collectively is brought into contact with the requisitevinyl aromatic monomer based on 1 million parts of the monomer. Themethod may, in other aspects of the invention, comprise the step ofheating the monomer and, in another aspect of the invention, the monomermay be distilled to remove impurities therefrom.

In another aspect of the invention, a vinyl aromatic monomeranti-polymerization composition is provided which comprises a liquidcarrier and dissolved or dispersed therein (A) a quinone methidederivative and (B) a phenol compound.

In another embodiment, the quinone methide derivative is 2,6-di-tert-butyl-4-benzylidene-cyclohexa-2,5-dienone.

In another aspect of the invention, the phenol compound (B) is2,6-di-t-butylphenol.

In another exemplary embodiment, a liquid carrier such as a non-polarorganic solvent is provided with the combined treatment (A) and (B)dissolved or dispersed therein.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In accordance with the invention, both a quinone methide derivative A)and a phenol compound B) are conjointly utilized to inhibitpolymerization of a vinyl aromatic monomer such as styrene.

The quinone methide derivatives generally have the formula:

wherein:

R₁ and R₂ are independently H, C₄ to C₁₈ alkyl; C₅ to C₁₂ cycloalkyl; orC₇ to C₁₅ phenylalkyl.

Preferably, R₁ and R₂ are tert-butyl, tert-amyl, tert-octyl, cyclohexyl,α-methylbenzyl or α,α-dimethylbenzyl; with tert-butyl, tert-amyl ortert-octyl most preferred.

R₃ is preferably aryl, or aryl substituted with C₁ to C₆ alkyl, alkoxy,hydroxy, nitro, amino, carboxy, or mixtures thereof.

Means for preparing these compounds may be found in U.S. Pat. No.4,032,547, the contents of which are wholly incorporated by reference toherein.

Preferably, the quinone methide derivative is 2,6-di-tert-butyl-4-benzylidene-cyclohexa-2,5-dienone.

The phenol compounds B) that may be utilized generally have the formula:

wherein R₄ and R₅ may be the same or different and are chosen fromC₁-C₂₀ alkyl, C₁-C₃₀ alkaryl and substituted C₁-C₃₀ alkaryl, R₆, whenpresent, is selected from C₁-C₂₀ alkyl, thiophenol, substitutedthiophenol, C₁-C₄₀ alkanoic acid ester, C₁-C₃₀ alkaryl, substitutedC₁-C₃₀ alkaryl, C₁-C₆ alkylamino, C₁-C₆ alkoxy, amine, polynuclear aryland substituted polynuclear aryl.

At present, the preferred phenol B) compound is 2,6-di-t-butylphenol.Exemplary phenols include 2,6-dipropylphenol, 2,6-diethylphenol and2,6-dimethylphenol. Also mentioned as exemplary are the hindered phenolsin accord with the above formula wherein R₄, R₅ and R₆ are all present.These include:

-   -   2,6-di-t-butyl-4-methylphenol    -   4,4′-thiobis-(6-t-butyl-2-methylphenol)    -   octadecyl 3-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate    -   4,4′-methylenebis(2,6-di-t-butylphenol)    -   1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene    -   2,6-di-t-butyl-α-dimethylamino-p-cresol    -   2,6-di-t-butyl-4-secbutylphenol    -   2,2′-methylenebis(4-ethyl-6-t-butylphenol)    -   2,2′-methylenebis(4-methyl-6-t-butylphenol)    -   2,2′-methylenebis(6-(1-methylcyclohexyl)-p-cresol; and    -   2,2′-methylenebis(4-methyl-6-cyclohexylphenol)

The compositions of the present invention are effective at inhibitingpolymerization of vinyl aromatic monomers under processing conditions.These processing conditions include but are not limited to preparation,purification, distillation and vacuum distillation processes.

Styrene, for example, is typically processed at temperatures between 95°C. and 125° C. The compositions of the present invention are effectiveat inhibiting the polymerization of styrene over this range oftemperatures.

The vinyl aromatic monomers that are treated by the compositions of thepresent invention include but are not limited to styrene, bromostyrene,divinylbenzene, and α-methylstyrene. The compositions of the presentinvention are particularly efficacious at inhibiting the polymerizationof styrene monomer.

The total amount of quinone methide derivative A) and phenolic compoundB) used in the methods of the present invention is that amount which issufficient to inhibit polymerization of vinyl aromatic monomers. Thisamount will vary according to the conditions under which the vinylaromatic monomer is being processed, contaminants in the system and thetemperature of the system. At higher processing temperatures and highermonomer contamination, larger amounts of the inhibiting composition arerequired.

For purposes of the present invention, the term “effective inhibitingamount” is that amount which is effective at inhibiting vinyl aromaticmonomer polymerization. Preferably, this amount ranges from about 1 partto about 10,000 parts of quinone methide derivative and phenoliccompound, collectively, per 1 million parts of monomer. Most preferably,this amount will range from about 1 to 1,000 parts per million partsmonomer.

Accordingly, it is possible to produce a more effective vinyl aromaticmonomer polymerization inhibiting treatment than is obtained by the useof either compound by itself when measured at comparable treatmentlevels. This synergism or enhanced activity between components allowsfor the concentration of each of the components to be lowered and thetotal quantity of polymerization inhibitor required, particularly athigher temperatures, may be lowered while achieving a commensurate levelof polymerization inhibition.

As such, the weight ratio of quinone methide derivative to phenoliccompound will generally range from about 90:10 to about 10:90. Mostpreferred is a weight ratio of about 70:30.

The compositions of the present invention can be introduced into thevinyl aromatic monomer by any conventional method at any point of theprocessing system, either as separate and individual ingredients or as acombination of ingredients.

The compositions of the present invention may be added to the vinylaromatic monomer as either a dispersion or as a solution using asuitable liquid carrier or solvent. Any solvent that is compatible withthe individual ingredients of the composition and the vinyl aromaticmonomer to be treated may be employed. It is often desirable to dissolvethe inhibitors in the monomer to which the inhibitor is being added toavoid introducing additional impurities in the monomer. Exemplary liquidcarriers include non-polar organic solvents, such as heavy aromaticnaphtha and xylene.

The method of the present invention can control the fouling ofprocessing equipment, such as the equipment used in separation andpurification processes of styrene monomer, which is due to or caused bythe polymerization of the monomer. The instant invention may be used asboth a process inhibitor, which is employed during preparation andprocessing (e.g., employing heat) of the styrene monomer (i.e.,ethylbenzene), and as a product inhibitor, which is combined with thestyrene monomer in order to inhibit polymerization during storage andhandling.

The invention will now be described in conjunction with the followingexamples which should be viewed as being illustrative of the inventionand should not be deemed to limit the invention in any manner

EXAMPLES

The effect of a combined treatment of QM (Quinone Methide) and2,6-di-tert butyl phenol on the thermal polymerization of styrene at115-120° C. was evaluated by comparing polymer formation utilizing thefollowing procedure.

A 250 ml RB flask-equipped with an Ar gas inlet, water cooled condenserand sample outlet was charged with 110 ml of styrene and the candidatepolymerization inhibitor(s). [600 ppm (w/v)]. The flask was purged withArgon for 10 minutes. The flask was then immersed into an oil-baththermostatically controlled at 115-120° C. and heated with purging Arcontinuously. Once the temperature reached 115° C., the stop clock wasstarted and this time was considered as time zero. About 5 ml of thesample was removed from the flask at varying time intervals for up to 4hours and measured precisely before pouring into about 40 ml methanol toprecipitate out the styrene polymer. The precipitated polystyrene wasfiltered with a gas membrane filter that was pre-weighed before use. Thepolymer was dried at 100° C. and weighed.

Styrene Polymerization Results are Shown in Table 1.

TABLE I Amount of polystyrene formation at 115-120° C. as a function oftime for a resultant retarder dosage of about 600 ppm with differentratios of 2,6-tertiary butyl phenol (DTBP) and QM. Polymer wt. in mgQM + 2,6- QM + 2,6- QM + 2,6- QM + 2,6- QM + 2,6- QM + 2,6- QM DTBP DTBPDTBP DTBP DTBP DTBP Time in (pure) (569.3 ppm: (479.98 ppm: (450.24 ppm:(420.22 ppm: (387.3 ppm: (300.07 ppm: minutes 600 ppm 31.68 ppm) 119.07ppm) 149.94 ppm) 180.45 ppm) 210.78 ppm) 298.71 ppm)  60 19.58 15.8012.60 10.1 6.8 13.60 16.20 120 42.85 34.90 24.80 23.2 15.0 29.00 34.50180 70.75 59.00 41.10 40.1 26.4 50.30 58.60 240 105.85 89.20 60.60 60.540.0 76.20 90.30 QM 100.00 94.73 80.12 75.02 69.96 64.76 50.11 (%) 2,6-0.00 5.27 19.88 24.98 30.04 35.24 49.89 DTBP (%)

While we have shown and described herein certain embodiments of theinvention, it is intended that these be covered as well as any change ormodification therein which may be made without departing from the spiritand scope of the invention as defined in the appended claims.

1. A method for inhibiting the polymerization of vinyl aromatic monomercomprising adding to said monomer an effective polymerization inhibitingamount of a compound comprising (A) a quinone methide derivative havingthe formula

wherein: R₁ and R₂ are independently H, C₄ to C₁₈ alkyl; C₅ to C₁₂cycloaklyl; or C₇ to C₁₅ phenylalkyl, and R₃ is aryl, or arylsubstituted with C₁ to C₆ alkyl, alkoxy, hydroxy, nitro, amino, carboxy,or mixtures thereof; and (B) a phenol compound having the formula

wherein R₄ and R₅ may be the same or different and are chosen fromC₁-C₂₀ alkyl, C₁-C₃₀ alkaryl and substituted C₁-C₃₀ alkaryl; R₆ isselected from C₁-C₂₀ alkyl, thiophenol, substituted thiophenol, C₁-C₄₀alkanoic acid ester, C₁-C₃₀ alkaryl, substituted C₁-C₃₀ alkaryl, C₁-C₆alkylamino, C₁-C₆ alkoxy, amine, polynuclear aryl and substitutedpolynuclear aryl.
 2. The method as recited in claim 1 wherein the weightratio of A to B is about 90:10 to about 10:90.
 3. The method as recitedin claim 2 wherein said quinone methide derivative A) is2,6-di-tert-butyl-4-benzylidene-cyclohexa-2,5-dienone.
 4. The method asrecited in claim 3 wherein said phenol compound B) is 2,6-di-tert-butylphenol.
 5. The method as recited in claim 4 wherein from about 1,-10,000ppm of A and B collectively is brought into contact with said vinylaromatic monomer, based on 1 million parts of said vinyl aromaticmonomer.
 6. The method as recited in claim 5 wherein said vinyl aromaticmonomer comprises styrene monomer.
 7. The method as recited in claim 5wherein the weight ratio of A:B is about 70:30.
 8. The method as recitedin claim 7 further comprising the step of heating said styrene monomer.9. The method as recited in claim 7 further comprising the step ofdistilling said styrene monomer to remove impurities therefrom. 10.Vinyl aromatic monomer anti-polymerization composition comprising aliquid carrier and dissolved or dispersed therein A) a quinone methidederivative having the formula

wherein:. R₁ and R₂ are independently H, C₄ to C₁₈ alkyl; C₅ to C₁₂cycloaklyl; or C₇ to C₁₅ phenylalkyl, and R₃ is aryl, or arylsubstituted with C₁ to C₆ alkyl, alkoxy, hydroxy, nitro, amino, carboxy,or mixtures thereof; and (B) a phenol compound having the formula

wherein R₄ and R₅ may be the same or different and are chosen fromC₁-C₂₀ alkyl, C₁-C₃₀ alkaryl and substituted C₁-C₃₀ alkaryl; R₆ isselected from C₁-C₂₀ alkyl, thiophenol, substituted thiophenol, C₁-C₄₀alkanoic acid ester, C₁-C₃₀ alkaryl, substituted C₁-C₃₀ alkaryl, C₁-C₆alkylamino, C₁-C₆ alkoxy, amine, polynuclear aryl and substitutedpolynuclear aryl.
 11. Vinyl aromatic monomer anti-polymerizationcomposition as recited in claim 10 wherein A) and B) are present in aweight ratio of about 90:10 to about 10:90.
 12. Vinyl aromatic monomeranti-polymerization composition as recited in claim 11 wherein saidquinone methide derivative A) is 2,6-di-tert-butyl-4-benzylidene-cyclohexa-2,5-dienone.
 13. Vinyl aromatic monomeranti-polymerization composition as recited in claim 12 wherein saidphenol compound B) is 2,6-di-tert-butyl phenol.
 14. Vinyl aromaticmonomer anti-polymerization composition as recited in claim 12 whereinsaid liquid carrier comprises a non-polar organic solvent and wherein A)and B) are both dissolved in said solvent.
 15. Vinyl aromatic monomeranti-polymerization composition as recited in claim 14 wherein saidnon-polar organic solvent comprises heavy aromatic naphtha or xylene.16. Vinyl aromatic monomer anti-polymerization composition as recited inclaim 10 further comprising styrene monomer.